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How Caves Form

Caves are cavities that form in hillsides, in cliff walls, and underground, and are accessible at Earth's surface. Typically, they form in soft, fractured, or soluble rock as a result of natural mechanical and chemical processes that continue over thousands of years. This interactive activity from NOVA offers animated explanations for how ocean waves, lava, slightly acidic groundwater, and, in a supporting role, microbes account for the variety of cave formations on Earth.

One-seventh of the world's land surface lies atop landscapes characterized by grooved rock pavements, streams that disappear underground, and scattered circular depressions called sinkholes. This type of topography is known as karst terrain—terrain in which land formations result primarily from groundwater dissolving minerals in the rock.

Whereas ocean waves carve sea caves out of rock faces through unrelenting physical action, and lava caves are the empty tubes where molten rock once flowed, most underground caves are formed by the dissolution of soluble rock by water. Slightly acidic groundwater does its damage by slowly dissolving limestone. The result is a subterranean environment consisting of caverns and networks of smaller, interconnecting chambers.

Caves offer a unique habitat for rare flora and fauna, whose study has contributed to our knowledge of biological adaptation and evolution. One type of animal that has adapted to life in the dark zones of caves is the sightless troglobite, some species of which have evolved extra-long sensory antennae atop their heads for finding their way around.

Caves, and, more broadly, karst terrain, also hold much in the way of human, economic, and scientific value. Their underground springs provide fresh water for domestic and agricultural use, and many are mined for mineral resources such as marble, salt, and even guano, or bat excrement, a source of phosphate and an effective organic fertilizer. Additionally, several million visitors pass through karst regions each year to witness their aesthetic beauty and to simply explore.

For scientists, caves offer opportunities for different kinds of exploration. In medical research, the study of extremophiles—organisms that that live in conditions outside of a normal range of temperature or pH balance—is a growing area of interest. Several cave dwelling species are presently being tested for their ability to combat cancer and malaria.

The unregulated and relatively fast flow of water through caves and other karst features provides little opportunity for natural filtering to occur. As a result, karst systems are especially sensitive to environmental disturbances, such as siltation and disease transmission. Because the integrity of any karst system is dependent on both water and land, an effective conservation policy is one that provides comprehensive protection of natural water, airflow, and humidity conditions through caves and their wider catchments.